Most modern electric circuits are formed as "printed" circuits on a substrate. In the field of circuit fabrication, one of the more critical functions is the ability to control the size (length, width, height), or critical dimensions (CD), of features such as contact holes and trenches which form the circuits. Any deviation in the dimensions of a feature can adversely impact on the performance of the resulting circuitry.
As critical features become smaller, metrology (the science of measurement) using imaging tools such as optical microscopes, scanning electron microscopes (SEM), atomic force microscopes (AFM), and other similar tools becomes more challenging. The challenge is especially acute for contact holes and trenches with diameters less than 0.25 .mu.m. For example, reduced collection efficiency of SEM secondary electrons from such contact holes and trenches can provide false edge sharpening. For this and other reasons, current measurement methodologies are poor in their correlation wish cross section results taken from actual samples. Reduced collection efficiency also may interfere with obtaining critical information from the bottom of a contact hole.
Optical microscopes, SEM, and AFM are all instruments used to obtain an enlarged image of a small object such as semiconductor features. An optical microscope generally has a light source, a condenser, an objective, and a recording device such as a photoelectric tube or a photographic plate. The optical microscope is limited by the wavelengths of the light used and by the materials available for manufacturing the lenses.
A SEM is an electron instrument that builds up its image as a time sequence of points in a manner similar to that used in television. The imaging method of the SEM allows separation of the two functions of a microscope: localization and information transfer. The SEM uses a very fine probing beam of electrons which sweeps over the specimen to emit a variety of radiations. The signal, which is proportional to the amount of radiation leaving an individual point of the specimen at any instant, can be used to modulate the brightness of the beam of the display cathode ray tube as it rests on the corresponding point of the image. In practice, the points follow one another with great rapidity so that the image of each point becomes the image of a line. The image can also be recorded in its entirety by allowing the point-by-point information to build up in sequence on a photographic film.
The AFM waveform may be a more accurate representation of the actual topology of the specimen, than that produced by the optical microscope or SEM, because an AFM is analogous to a stylus that runs along the surface of the specimen. When the tip of the AFM encounters a feature, the tip rises; when the surface goes down, the AFM records the actual vertical motion alone the axis, mimicking the actual motion of the tip across the surface. Instead of using an electron beam, the AFM actually monitors the vertical motion as a function of time analogous to the stylus of an analog disc player. The AFM tip is more sensitive than a stylus, however, and senses the sur ace electronically. Because the tip does not actually touch the surface of the specimen, no risk of damage arises. The AFM provides a waveform which is similar to a topology path. The common step in both SEM and AFM measurement is the identification of characteristic components of the waveform generated by the tool.
Metrology tools typically report only the diameter (or line width) of a particular feature. Shown in FIG. 1 is a CD SEM image 10 of a contact hole 12; the diameter of contact hole 12 is identified between the arrows "A." Shown in FIG. 2 is a CD SEM image 14 of a trench 16; the line width of trench 16 is identified between the arrows "B." The measurement results from analyzing the waveform for edge information. Sometimes subroutines are added to further explore the waveform information content around the edge to determine if a resist foot is present. This approach is most robust for an isolated line where a determination of the surrounding signal baseline can be established. When applied to contact holes and dense structures like nested lines, however, difficulties often arise in implementing this approach.
Furthermore, the conventional approach does not reliably provide information concerning whether the feature (contact hole or trench) is open or closed. Such information is an important concern and could be determined from the waveform in many cases. As critical features become smaller, more information is needed than simply the line width or diameter. Information about the status of the contact hole or trench proves valuable and, in some cases, is more important than the line width or diameter.
The deficiencies of the conventional measurement methodologies show that a need still exists for a method which will accurately and reliably characterize a semiconductor feature such as a contact hole or trench. To overcome the shortcomings of the conventional measurement methodologies, a new method is provided. An object of the present invention is to provide a method which accurately, reliably, and with high quality measures the width or diameter of a feature.
Another object of the present invention is to provide a measure of the collection efficiency of the CD SEM used to collect waveform data from the feature for characterization of the feature. A related object is to use the measure of the collection efficiency to determine whether information from the bottom of the feature is meaningful. When the information is found to be meaningful, yet another object is to use the information to evaluate the status of the feature as open or closed. Finally, a collective object of the present invention is to provide a measure of quality not yet achieved in conventional CD SEM metrology by reporting collection efficiency, whether the feature is open or closed, and an accurate diameter measurement.